Fundamentals of Manufacturing

limits

the maximum and minimum values for a manufactured parts dimensions

tolerance

permissible variation in size or dimensions of a manufactured component

tolerance zone

difference between maximum and minimum allowable tolerance values

interchangeable manufacturing

parts are made nearly alike, so they can be used with the same functionality as part of a given mechanism

unilateral tolerance

deviation expressed in only one direction from the basic value

bilateral tolerance

deviation from basic value extends in both directions, can be equal or unequal

limit tolerance

deviation isn’t specified, only upper and lower limits are stated

chain dimensions

individual dimensions are cumulative, so tolerances added to those dimensions are also cumulative

running dimensions

dimensions are all related to a datum surface, tolerance is the same for each running dimension, so overall tolerance is the same as for each running dimension

International Tolerance (IT) ranges

set by ISO 2768 standard, grades each manufacturing process, then use that grade and the base dimension to determine the tolerance range

fit

property of mechanical assemblies that determines the manner according to which manufactured components interact with one another

clearance fit

determines spacing between two mating parts, e.g. free spinning shaft in a non-moving hole

interference fit

mating parts have intersecting volumes, requiring their deformation for the desired assembly, e.g. rivets in an aircraft fuselage

transitional fit

limits of the fit are such that they may either be clearance or interference, functionality of the assembly is determined by other parameters

allowance

difference between the maximum and minimum material limits of mating parts, calculated by subtracting the upper limit of the shaft dimension from the lower limit of the hole dimension

basic size

intended theoretical dimension from which the limits are derived

manufacturing tolerance specification - hole basis

hole is machined to the lower limit specified and the shaft is the machined to meet the required fit

manufacturing tolerance specification - shaft basis

shaft is machined first, usually used when multiple components need to fit onto a singular shaft, shaft is manufactured to its upper limit and then holes will be machined to create the required fit

Metric engineering fits

done to ISO 286-1 standard, most used subclasses are: 5 clearance fits, 2 transitional fits and 3 interference fits

clearance fit subclasses - shaft smaller than the hole

loose running fit

fit with largest clearance for applications where accuracy isn’t essential and the operating environment induces contamination, provides wide tolerances and allowances, e.g fits exposed to corrosion, dust contamination and high temperature cycles

free running fit

large clearance, applicable where the accuracy of the matching components doesn’t prompt special requirements, leaves room for movement in environments with heavy temperature fluctuations, high running speed and heavy plain bearing pressures, e.g. assemblies where films of lubricant must be maintained, shaft and plain bearing fits

close running fit

suitable for applications requiring small clearances and moderate accuracy, good for medium speeds and pressures, e.g. machine tools, CNC spindles

sliding fit

leave minimal clearance, used when high accuracy is needed alongside ease of assembly/disassembly, parts turn and slide freely, e.g. guiding of shafts, slide valves, machine tools, clutch discs, sliding gears

locational clearance fit

provides smallest clearance for high accuracy requirements, assembly doesn’t require force, matching parts can rotate and slide freely with lubrication, provides a snug fit for static parts, e.g. roller guides and guiding of shafts

transitional fit subclasses - shaft is slightly larger than the hole or clearance fit with small room for movement

similar (locational transitional) fit

used when accurate location is necessary, small interference or small clearance is suitable, e.g. wheel hubs, gears, pulleys, bearings

fixed fit

similar to similar fit (used when accurate location is necessary, small interference or small clearance is suitable) but assembly carried out with some light force, e.g. armatures on shafts, driven bushes

interference fit subclasses - larger shaft with a smaller hole

press fit

provides accurate alignment with minimal interference, assembly achieved with cold pressing, e.g. hubs, bushings, bearings

medium drive (driving) fit

requires higher assembly forces for cold pressing, or use hot pressing, produces a tighter fit, e.g. permanent mounting of gears and shafts

forced fit

high interference fit, assembly requires large temperature difference (hot hole and frozen shaft) to ensure forced shrink fit of the parts together, disassembly risks destruction of mating parts, e.g. gears, shafts

machining

manufacturing process involving the controlled removal of material - subtractive

CNC machining

Computer numerical control, automated control of tools via pre-programmed instructions to manufacture components

axis and dimensions for machining

relating to the number of degrees of freedom the toolhead can move in, generally cartesian coordinate system

fixturing

how the stock is held in the machine, helps to repeat things if the stock is held in the same way

stock

raw material the part will be cut from, produced in regular shapes, should be larger than the desired final part

work offset (G54)

reference point that bridges the gap between the CAM setup and the real world, good practice to choose a fixed repeatable point e.g. corner

incremental positioning (G91)

program dimensions or positions are given from the current point, how far away from where you want to be

absolute positioning (G90)

all dimensions or positions are given from a single reference point on the job, how far way from where it started

tool offset

required to ensure the machined part is the correct dimension, based on geometric information about the tool, offset lists, wear compensation

tools

end mills, slot mills, facing, tapping, boring, drilling, chamfering, inserts

radial tool offsets

climb milling (left)

tool “climbs” along the material, large initial cut which thins as it progresses, rigid machine with backlash eliminators required, generally gives a better finish on most metal

conventional milling (right)

rotating away from the material, thin cut getting thicker, manual or less rigid machines, can give better finishes on other materials

roughing

remove large amounts of material quickly creating a rough shape, high tool loading, tooling witness marks may be present, tool deflection, don’t rough to final size (always leave extra stock)

finishing

takes thin passes, achieves desired tolerance and mirror surface, not always required - need to think about the use of the part

slotting

engages full diameter of the tool

side cutting (shouldering)

doesn’t cross the centreline

slot drills

1 tooth is longer than the other which helps provide continuous cutting, allows for plunging into material

drilling

a plunge operation

boring

rotational action for making precise round holes

tapping

engages a canned cycle that matches spindle speed and vertical feedrate to the pitch of the tap being used, stops and reverses the spindle direction to retract

entrances and exits from materials

step over

radial depth of cut

step down

axial depth of cut

feed

rate of material fed in or rate of cutter advancement, expressed in mm/min

speed

spindle speed, rotational velocity of the tool (milling) or the workpiece (turning), expressed in r/min

facing

prepares raw stock for machining, regularising or squaring up, fast planar material removal, good for creating datum faces

contour

simplest operation, can be shoulder or slot milling, cuts a line along a contour of a fixed height, typically a finishing operation, multiple depths

pocket

classical roughing or finishing operation, contouring operation thats offset radially multiple times, toolpaths parallel to geometry, clearing areas

adaptive clearing

similar usage as pocketing, constant tool engagement - reduces shock loading of tool, less prone to breakages - longer tool life and reduced costs, creates a more complex toolpath - difficult to proram manually

chamfer

“breaking the edge”, uses a non-90 degree cutter, typically 45 or 60, an offset contour toolpath, eliminates sharp edges, safer to handle and aids in assembly

G-code

universal standard, commands that tell the machine what to do

G00

rapid positioning

G01

linear interpolation

G02

circular interpolation clockwise

G03

circular interpolation counterclockwise

G04

dwell

G17

XY plane selection

G18

ZX plane selection

G19

YZ plane selection

G20

unit selection - imperial

G21

unit selection - metric

G40

cutter compensation - cancel

G41

cutter compensation - left

G42

cutter compensation - right

G81

drill cycle - spot boring

G82

drill cycle - counterboring

G83

drill cycle - peck

G84

tapping cycle

M-code

differs between machine, but focuses more on the program rather than the actual machining process

M00

program stop

M01

optional stop

M02

end of program

M03

spindle start - forward clockwise

M04

spindle start - reverse counterclockwise

M05

spindle stop

M06

tool change

M08

flood coolant on

M09

coolant off

M30

end of program (return to top of memory)

T

tool (followed by number or description)

F

feedrate (m/min)

S

spindle speed (rpm)

conventional machining

hard, sharp tools, in contact with workpiece, human driven input

chips

made in some form by removing material to make the part

causes of wear

excessive heat (tempering), abrasion (dulled edges), build up on tool edge (galling), clogging of flutes (galling), chipping of tool (overloading), workpiece work hardening (rubbing and glazing)

effects of wear

damaged tools, bad surface finish, reduced efficiency

twist drill

rapid material removal, only cuts axially, ‘round’ but not precision holes, tip angle - 118 typical, 140 for harder metals, 60 for softer woods, cutting lip cuts and chisel lip deforms, helix angles - slow, standard, quick